1. Field of the Invention
The invention is directed to liquid crystal compositions and, in particular, to liquid crystal compositions which are useful in multiplexed twisted nematic display devices, with the nematic phase existing from at least about -20.degree. C. to at least about 50.degree. C.
2. Description of the Prior Art
Liquid crystal displays are now being employed in numerous commercial applications as electro-optical indicator systems and many types of these systems have now been developed, including those utilizing the twisted nematic field effect. For display devices based on the twisted nematic field effect, it is now recognized that the liquid crystal composition should exhibit positive dielectric anisotropy, a mesomorphic temperature range, including room temperature, of at least .ltoreq.0.degree. C. to .gtoreq.40.degree. C., a birefringence of about 0.13 or greater, a low viscosity for good response times, and be preferentially aligned on supporting substrates to constitute an initially twisted structure. A number of methods are known in the prior art to produce suitable alignment of liquid crystal materials, as is apparent to the artisan.
In addition to the above-mentioned desirable characteristics, the liquid crystal material should, for long term device reliability, be an eutectic composition, have high purity, and exhibit good chemical, photochemical and electrochemical stability.
Where the number of addressed elements in a liquid crystal display is small, separate connections are made to each element, i.e., one driver per element. In this "static drive" mode, zero voltage is applied to an unselected (OFF) element, and a finite voltage greater than the threshold (V.sub.Thr), to selected (ON) elements. Such addressing may be used in simple wristwatch displays (4 to 6 digits, 7 segments per digit) where the threshold voltage is typically &lt;1.5 V and the operating voltage is 3 V.
Many liquid crystal compositions are known that meet the majority of the above requirements and may be obtained commercially, e.g., cyanobiphenyl compounds may be obtained from B.D.H. Chemicals Ltd., Poole, Dorset BH12 4NN, England.
For complex, multi-element displays, e.g., calculator, alpha-numeric, dot matrix displays, it may not be possible to make separate connections to each element and some form of multiplexing or matrix addressing (time sharing) is required. Liquid crystal displays in general, and twisted nematic displays in particular, change their optical properties in response to the RMS (root mean square) value of the alternating voltage. In this "dynamic drive" mode, a finite voltage (V.sub.off) is applied to unselected (OFF) elements and a higher voltage to the selected (ON) elements. To avoid "crosstalk", where an unselected (OFF) element appears visible, V.sub.off is set below the threshold voltage (V.sub.Thr). For conventional multiplexing, there is a maximum value of the ratio V.sub.on :V.sub.off dependent upon the number of scanned rows (n), ##EQU1## The relationship between V.sub.on :V.sub.off and number of scanned rows (n) may be seen as follows:
______________________________________ n ##STR5## ______________________________________ 2 2.414 3 1.932 4 1.732 5 1.618 6 1.543 7 1.488 8 1.447 9 1.414 10 1.387 ______________________________________
To achieve a decreased duty cycle (i.e., greater number of scanned rows), the liquid crystal composition and display construction must be chosen to reduce the V.sub.on V.sub.off ratio, since "crosstalk" does not allow the V.sub.off voltage to be scaled to increase the V.sub.on voltage and give equivalent contrast at a lower duty cycle.
The threshold voltage (V.sub.Thr), which determines the V.sub.off voltage is not a single value for a given liquid crystal composition and display construction, but varies as a function of the angle of viewing and temperature. Furthermore, the twisted nematic display is characterized by a shallow electrooptic transmission curve.
The effects of liquid crystal material birefringence (.DELTA.n), cell spacing, and surface alignment on the electrooptic characteristics of a twisted nematic display indicate that the V.sub.on :V.sub.off ratio is minimized by using a low birefringent material, a thin cell spacing, and a near zero tilt surface alignment. Furthermore, the threshold voltage (V.sub.Thr) and the sharpness of the contrast curve is determined by the dielectric anisotropy and the ratios of the three elastic constants (splay k.sub.11, twist k.sub.22, bend k.sub.33). A favorable combination of these parameters leads to a sharp "knee" in the electro-optic transmission curve and thus a lower V.sub.on :V.sub.off ratio.
The temperature dependence of the threshold, an intrinsic property of the liquid crystal composition, varies from class to class of materials. Where temperature compensation of the addressing voltages is not done, the V.sub.off voltage is set at the highest operating temperature to avoid crosstalk at lower temperatures and is the most significant parameter in producing low duty cycle, multi-element displays.
Liquid crystal compositions to effect a reduction in the ratio V.sub.on :V.sub.off have recently been formulated from mixtures of positive (Np) and negative (Nn) dielectric anisotropy components, rather than purely positive materials. A few of the latter type of compositions exhibit satisfactory multiplexing characteristics, e.g., pure cyanobiphenyls/terphenyl mixtures, known as "E26M" and "E25M" which are available from B.D.H. Chemicals Ltd., Poole, Dorset BH12 4NN, England. However, such compositions, whilst having quite sharp threshold characteristics, do exhibit a large temperature-threshold variation. Liquid crystal compositions of cyanophenylcyclohexanes (Np) and esters (Nn) exhibit low temperature-threshold dependence, but do not have sharp electro-optic transmission curves, e.g., compositions known as "ZL1 1216" and "ZL1 1253" which are available from E.M. Laboratories, 500 Executive Boulevard, Elmsford, New York 10523. Liquid crystal compositions of cyanobiphenyls (Np) and benzoate esters (Nn) have been used in low duty cycle (1 in 7) multiplexed displays particularly for displays where temperature compensation of the addressing voltages is done (see, for example K. Odawaru et al, "An 80 -Character Alphanumeric Liquid Crystal Display System for Computer Terminals", S.I.D. Digest, paper number 13.6, 1979).
To compare the "degree of multiplexing" of the different types of liquid crystal compositions, a figure of merit may be defined when the compositions are examined under the same conditions of cell spacing, surface alignment, polarizer combination, addressing waveform, illumination and detection system. The merit number may be defined by the ratio of the minimum threshold voltage to the voltage for a given transmission (contrast or contrast ratio) at a particular position of viewing of the display. This has been done for a number of presently available "multiplexing mixtures". (See E. P. Raynes, "Recent Advances in Liquid Crystal Materials and Display Devices", IEEE/SID Biennial Display Research Conference pp. 8-11, 1979.) Mixtures of different classes of liquid crystal materials, particularly those of positive and negative dielectric anisotropy, produced compositions with enhanced multiplexing capability.
It is known that some liquid crystal compositions of purely nematic components exhibit induced smectic behavior, thus decreasing the useful operating temperature range of the composition in a twisted nematic display device. Whilst components from a single class of compounds may show this behavior, (e.g., mixtures of higher homologues of cyanobiphenyls), mixtures of different classes of compounds exhibit this behavior most readily, particularly Np and Nn materials, e.g., cyanobiphenyls and benzoate esters. Specific examples of mixtures of terminal nonpolar and polar liquid crystals are given in B. Engelen et al, Molecular Crystals and Liquid Crystals, Vol. 49 (letters), pp. 193-197, 1979, and Ch. S. Oh. Molecular Crystals and Liquid Crystals, Vol. 42, 1, 1977. Such behavior limits the choice and composition of components (see U.S. Pat. No. 4,147,651) for an adequate temperature range of operation of a twisted nematic display device.
The degree of alignment of the liquid crystal composition is extremely important to producing a twisted nematic display, both the electro-optical performance, and the longevity of the display device being critically determined by this interface. Many alignment methods are known in the prior art, both organic and inorganic layers, and different classes of liquid crystal materials are oriented to greater or lesser degrees by these surfaces. Of particular interest are alignment surfaces that will withstand high temperatures as seen in sealing display cells with glass frit to enable fabrication of hermetic packages. As is now well-known, silicon monoxide may be deposited to give a suitable alignment surface for multiplexed displays.
It is the inventor's experience that such surfaces do not align, over broad temperatures, many liquid crystal compositions incorporating prior art mixtures of cyanobiphenyls and benzoate esters, particularly those that give good multiplexing behavior on other surfaces, e.g., polyvinyl alcohol rubbed surfaces. This is seen as a major drawback to the use of such compositions in long life glass frit sealed displays.
To the nematic phase of liquid crystal compositions used in the twisted nematic display, it is common to add a small percentage of an optically active component which may or may not be a cholesteric liquid crystal. The resultant long pitch cholesteric liquid crystal composition has a unique sense of twist in the display cell and eliminates the possible existence of reverse twist areas.
Nonetheless, notwithstanding the formidable selection of mixtures of active compounds described in the prior art, it is believed that the prior art teachings nowhere teach, nor do they render obvious, the particular compositions of the present invention which have advantageous liquid crystal display characteristics.